It’s a rule, right? No project is complete without three unplanned trips to the hardware store. Needing screws for an electrical box can absolutely chew up one of those trips. But I can at least make it easier. Let’s talk about electrical box screw size, because in the United States at least, it’s almost certainly one of three sizes.
The most common electrical box screw size is 6-32. However, for many applications, 6-32 is too light duty, so we use an 8-32 or even a 10-32 instead. The thread pitch, however, is always 32.
Electrical box screw size for outlets and switches
The most common electrical box screw size is 6-32, and you can take advantage of this to try to save yourself a trip, or buy yourself some time to reduce the number of trips. The most common size inside an electrical box is 6-32, and that’s also the same size screw that you use to hold cover plates on outlets and switches. So in a pinch, you can borrow a 6-32 from a light switch to verify the size. Use an oval-head screw for cover plates. Inside the box, use round-head screws to get the best electrical contact and prevent arcing.
If the 6-32 is just slightly too small, then you need an 8-32. If the 6-32 is noticeably too small, you need a 10-32.
A 6-32 screw is .138 inches in diameter. So in a pinch, you can use a drill bit to measure. If a 1/8-inch drill bit fits with just a little room to spare, you need a 6-32.
The length can vary, but I find 1/2-inch to 1-inch length screws almost always fit, except in the case of GFCI outlets. It’s safest to go with 5/16-inch for GFCI. GFCIs often go into tile walls and the shorter screws accommodate the tile.
On the other extreme, long screws can be helpful for dealing with boxes recessed deep into walls. I’ve sometimes had to use extension boxes and extra long screws to deal with that situation.
Electrical box screw size for junction boxes
Junction boxes usually use a heavier duty screw, an 8-32. Junction boxes often hold heavier light fixtures, so they need a screw that can hold a bit more weight. 8-32 is a very common size, at least. An 8-32 is .164 inches in diameter, which is a shade over 5/32. A 5/32 bit will not fit in place of a 6-32.
In this case, the length depends almost entirely on application. When hanging a light fixture, I like to replace the screws that came with the fixture with longer ones, as I find having some extra slack on the screws makes it easier to line the holes on the fixture up with the screws. If I have very little slack I have to get both of them right, but if I have a lot of slack, I can get one, then find the other. Then I tighten the screws.
And finally, ground screws
Ground screws are larger still, generally, 10-32, and generally 5/16″ long, but you can usually thread a longer screw in. By convention they are usually green, to indicate to everyone what they are. But in a pinch, a regular bare metal 10-32 screw will work for ground. It just has to be a conductor, and bare metal conducts electricity pretty well. Hex heads are common on ground screws. You can substitute a round head if you wish. Just don’t use a flat or oval head, as those types of heads won’t make very good contact with the wire.
I keep a small parts organizer containing various lengths of 6-32 and 8-32 screws, since both of them are so common. It’s saved me a lot of hassle since I bought that organizer and populated it. This assortment of 8-32s is a good buy for getting started. If you don’t do a lot of electrical work it can be overkill, but I find a lot of other things I’m interested in also use 6-32 and 8-32 screws extensively. Even vintage Erector sets used 8-32s and 6-32s. Really.